This invention involves various electrical devices and apparatus for sensing physical conditions, such as smoke and other particle detectors, acoustic intrusion alarms, and the like, particularly apparatus wherein the physical sensing means is powered in pulses. For example, some optical smoke detectors now include a clock which controls supply of power to a light source as well as to smoke sensor circuits which process the electrical response of a photocell to which the pulsed light is scattered by smoke. In some installations, once the smoke sensing circuits determine that smoke has reached a predetermined alarm level of density, it is desirable that the apparatus be latched into an alarm condition.
In such pulsed detection circuits there is a problem resulting from the possibility that transient voltage surges in the power lines to the detector, electromagnetic voltages surges induced in the detector, and other electrical interference will spuriously simulate the photocells response to smoke scattered light pulses. It has been proposed that the effect of transient electrical surges be nullified by integrating the detected voltage pulses until an alarm level is reached as described in U.S. Pat. No. 3,872,499. Such a method, however, suffers from the disadvantage that short bursts of interference will be falsely integrated along with temporary smoke signals to cause a permanently latched alarm. Also long bursts of interference can be integrated to alarm level in the complete absence of smoke signals.
It is the object of the present invention to provide a logic circuit for a condition senser which discriminates against spurious smoke signals and does not latch in to alarm condition until after an adequate sampling has been made of the smoke or other physical condition.